As I recall, regionals aren't always included in the BTS data for whatever reason? When I first started up my flight log last year, I backfilled a few gaps including international flights using planefinder.net, which was basically like FR24 with historical replay so if you knew roughly when a flight took off or landed you could move the map to that airport around the right time and see if you could catch it (I have a lot of free time on my hands ) but when I went back again more recently they appear to have severely limited that functionality. (although even that was a while ago, so possibly it's changed)

How long ago are you looking to go? If it's within 365 days then anyone with a FR24 account could look it up for you. Business plan goes back to 2 years.

Meanwhile, I sent messages to WN (Twitter), UA (contact us form) and DL (contact us form) a week or two ago asking if they had flight data (and making it clear that obv this was a low priority request given COVID and all)

WN replied almost immediately and happily sent a spreadsheet but only back to 2014 and only with RR activity so I could see when points were earned from flights but not any actual data. I asked if they had O&D info etc. and going further back and got a 2nd spreadsheet with flight numbers and airports but still only back to 2014. It's not that important so I left it at that.

UA replied back and said basically what's on the web site is what we can get you.

DL hasn't replied yet (totally fine, again this is super unimportant)